IMS involves separation, characterization, or identification of ions based on their transport through gases driven by electric field. In conventional IMS considered here, separation is based on the absolute mobility (K) at moderate field intensity.
As for any separation method, a major IMS performance metric is the resolving power (R) that determines the achievable feature resolution and peak capacity. The value of R in drift-tube (DT) IMS scales as the square root of drift voltage and has been raised by increasing that voltage, currently up to R˜170 (for singly-charged ions) at 14 kV. Further resolution gains along this path are impeded by the difficulty and cost of generating, isolating, and safely using voltages much above 10 kV. Conventional IMS was also implemented in “differential mobility analyzers” (DMA), where ions are filtered while pulled by a fixed electric field across a perpendicular high-speed gas flow in a narrow channel. The DMA resolving power also increases at higher voltages across the channel, and has been similarly limited to ˜80 (achieved at 10 kV).
One alternative to voltage increases is extending the ion residence in DTIMS using gas counter-flow (Loboda et al., J. Am. Soc. Mass Spectrom. 2006, 17, 691). While R˜40 attained in that system exceeds the “diffusion limit” defined by drift voltage for a stationary gas by fourfold, it is still much below the best DTIMS benchmarks, and achieving much greater R values is complicated by inevitable flow non-uniformity across the tube.
Another alternative is replacing a fixed electric field by time-dependent fields. The “cyclotron IMS” (Merenbloom et al., Anal. Chem. 2009, 81, 1482), where a potential gradient is switched (using individually addressed electrodes) to chase ions around a circular track, has reached an exceptional R˜400-600. However, that approach is complex to implement and has poor sensitivity because of large ion losses in successive turns around the loop.
In a different approach of traveling-wave (TW) IMS adopted in the IMS/time-of-flight MS instruments of the Synapt family, ions are separated while “surfing” an oscillatory field wave propagated along a tunnel using individually addressed electrodes. Here the losses are minimal thanks to RF confinement, but the resolving power has been low at approximately 10-40. Such performance has precluded many IMS applications, and improving the resolution in TWIMS to (at least) the level of frontline DTIMS systems is topical.
Another desired capability is effective selection of ions with K values within a certain range, to prevent the saturation of charge capacity of devices storing ions for pulsed injection into subsequent IMS/MS or MS stages. For example, the ion accumulation funnel at an ESI/IMS interface commonly fills up in <10% of the IMS separation time. This causes major ion losses and compresses the dynamic range, unless elaborate multiplexing schemes are implemented. Similarly, the dynamic range of ion trap MS platforms is limited by the ion trap charge capacity.
What is needed are ion mobility devices and methods that separate or filter ions with higher resolving power.